The following information is provided to assist the reader in understanding the technology described below and certain environments in which such technology can be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technology or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Infrared gas sensors use an energy source emitting infrared energy to detect the presence of an analyte in an environment being tested. In general, the analyte absorbs infrared energy of a certain wavelength and this absorption can be quantified to determine the concentration of the analyte in the test environment. Several embodiments of infrared fluid sensors or analyzers are discussed, for example, in U.S. Pat. Nos. 4,355,234, 4,598,201 and 4,902,896, assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference.
Infrared (IR) gas detection instruments in, for example, the oil gas petrochemical or the “OGP” industry may use one or two infrared sources and one or two infrared detectors in the detection scheme. For those using two sources, one is used to directly measure gas in the analytical path. Those with a second source use it only internal to the instrument for purposes of stability, to compensate for unwanted internal analyte or to compensate for aging or degradation of components in the instrument. IR gas detection instruments with a second source typically use one of two common configurations. In the first configuration, a wide band source “A” is positioned to measure the transmission of a short path of internal housing atmosphere using two narrow band sensors at a reference and analytic waveband. Another wide band source “B” is positioned to measure the transmission of an external analytical path employing the same two narrow band detectors. The second configuration also uses two sources and two detectors. Unlike the first configuration, however, in the second configuration, the two sources are narrow banded at a reference waveband and at an analytic waveband.
These configurations attempt to manipulate the transfer function such that when the target transmission of the external path is calculated, certain parameters subject to variation, such as detector responsivity, may be cancelled. However, implementation is complex and expensive, and many problems still remain. For example, the transfer function for both configurations indicates that the total gas path is the product of the transmissions of the internal path and the external path. In the case in long term deployment, the internal path often becomes contaminated with either the target gas or an unknown analyte as a result of normal outgassing of materials within the housing. When this happens, the total transmission of the external-internal path changes, making it nearly impossible to provide a correct estimate of the concentration of an analyte in the external path.